May 08 2018

Using Sweat to Determine the Number of Contributors?

Since 1986, when the first criminal case was solved through the use of DNA fingerprinting in the UK, investigating crimes became a little easier. DNA has become a common, and frequently critical component to naming a perpetrator. But what happens when biological samples taken fail to produce a full profile? A team at the University of Albany wondered if a different biological sample could be used – sweat.

written by: Tara Luther, Promega

Each person has several million sweat glands spread throughout the outer layer of their skin, and as sweat is the main way that humans regulate their body temperature, individuals are continually leaving trace amounts of sweat on items they touch. As DNA testing from sweat is only possible if skin cells are included in the fluids and oils of sweat, the researchers turned to the chemical makeup of sweat itself to provide answers.

The chemical make-up of a person’s sweat varies depending on their age, sex, diet, and activity level. As no two individuals will experience the same hormone levels or conditions at the same time, the researchers hypothesized that each sweat sample found would be unique to the person that left it behind. Based on this, they wondered if they could determine the number of different individuals present at a crime scene.

In order to test their hypothesis, the team looked at three different metabolites found in trace amounts of sweat: lactate, urea, and glutamate. Lactate is a byproduct of the carbohydrate metabolism of the body, and is one of the major components of sweat. Urea is the major end product created in the liver from ammonia formed during protein catabolism and amino acid deamination processes. The concentration of urea in sweat is dependent upon how much protein a person consumes, protein catabolism, and their kidney function. Glutamate is an amino acid that is formed during metabolism and is stored and produced in muscle tissue. Amino acid content varies depending on a person’s physiological condition and their biological sex. The team chose these metabolites, because both urea and glutamate possess an enzymatic assay that is fundamentally understood and the expected concentration ranges of these metabolites would be significantly different.

The research team first established which assays they’d use to test the various metabolites, and then measured the results found from 50 mimicked sweat samples they created and 25 authentic sweat samples taken from the forearms of volunteers. They looked at each data set in the mimicked and authentic samples to determine if each individual produced a unique combination of lactate, urea, and glutamate in sweat, and determined that the p-values for both the mimicked and authentic samples were <0.001 for each. The low p-value, indicates that a significant statistical difference between individuals when considering the responses from the three analytes, and the team concluded that it was true that compounds inherent in sweat could be used to differentiate individuals present at a crime scene.

While sweat can be used to determine how many different people were present at a crime scene, the researchers cautioned that samples should not be stored in a database for comparison. Unlike DNA, which remains constant, a person’s sweat make-up is ever changing based on their lifestyle, and could differ from day to day. Therefore, while these tests could determine the number of people present at the scene, it could not determine who was at the scene, and should be used in conjunction with traditional DNA testing and investigation procedures, and not as a replacement.

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